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A pyramid (from πυραμίς pyramís) is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilateral, or of any polygon shape. As such, a pyramid has at least three outer triangular surfaces (at least four faces including the base). The square pyramid, with a square base and four triangular outer surfaces, is a common version. A pyramid's design, with the majority of the weight closer to the ground and with the pyramidion at the apex, means that less material higher up on the pyramid will be pushing down from above. This distribution of weight allowed early civilizations to create stable monumental structures. Civilizations in many parts of the world have built pyramids. The largest pyramid by volume is the Mesoamerican Great Pyramid of Cholula, in the Mexican state of Puebla. For thousands of years, the largest structures on Earth were pyramid

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Hierarchical Micromold Structuring via 2PP Micro-Stereolithography

Ho-Yun Lee

Inspired by nature, a common way to create hydrophobic surfaces is by introducing hierarchical and fractal microstructures on surface. While plenty of studies have demonstratedcreating PDC (polymer derived ceramic) for several applications include SiC-fiber for composite materials and cantilever in ceramic MEMs, no research from the author’s knowledge is devoted to creating hydrophobic PDC surface by fabricate micro size array-like structure on PDC. Therefore, fabricate hydrophobic PDC surface is the main goal of the project. To achieve the idea, a novel approach is introduced and presented in the report. The approach combines micro molding of PDC and 3D printing hydrophobic surface. By exploiting micro-stereolithography base on Two Photon Polymerization, the project first printed various 3D structure including fractal Sierpinski tetrahedron, pyramid, biomimic arrays and QR codes. The printed photoresists show very well define structures by using OM characterization and the hydrophobicity of structure were verified by drop shape analyser. The designed micro molds were printed on the KOH etched mold for PDC molding. In the end, using this novel method, first ever micro-featured PDC were successfully replicated. Although the hydrophobicity for pyramid arrays on PDC is not yet verified, several improvements are provided to optimize future experiment.

2020

Designing high ductility in magnesium alloys

Rasool Ahmad, William Curtin, Zhaoxuan Wu, Binglun Yin

The thermally activated pyramidal-to-basal (PB) transition of (c + a) dislocations, transforming glissile pyramidal dissociated core structures into sessile basal dissociated ones, lies at the origin of low ductility in pure magnesium (Mg). Solute-accelerated cross-slip and double cross-slip of pyramidal (c + a) dislocations have recently been proposed as a mechanism that can circumvent the deleterious effects of the PB transition by enabling rapid dislocation multiplication and isolating PB-transformed sessile segments. Here, the theory for solute-accelerated cross-slip is revisited with an explicit atomistic derivation, is extended to include multiple very dilute solute concentrations, and various aspects of the theory are demonstrated computationally. DFT inputs to the theory for a wide range of new alloying elements are presented. The theory is validated by comparing predicted ductility to literature experiments for a range of alloys. The theory is then applied to predict composition ranges for ductility in rare-earth free ternary and quaternary dilute alloys. The wide range of new alloys predicted to be ductile can serve as a guide to experimental development of new ductile Mg alloys. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

PERGAMON-ELSEVIER SCIENCE LTD2019

Analysis of double cross-slip of pyramidal I < c plus a > screw dislocations and implications for ductility in Mg alloys

Rasool Ahmad, William Curtin, Zhaoxuan Wu

Solute accelerated cross-slip of pyramidal < c + a > screw dislocations has recently been recognized as a crucial mechanism in enhancing the ductility of solid-solution Mg alloys. In pure Mg, cross-slip is ineffective owing to the energy difference between the high energy pyramidal I and low energy pyramidal II < c + a > screw dislocations. A small addition of solutes, especially rare earth (RE) elements, can reduce this energy difference and accelerate cross-slip, thus enabling enhanced ductility. With increasing solute concentrations, the pyramidal I dislocation can become energetically favorable, which switches the primary < c + a > slip plane and alters the cross-slip process. Here, the transition path and energetics for double cross-slip of pyramidal I < c + a > dislocations are analysed in the regime where the pyramidal I dislocation is energetically more favorable than the pyramidal II. This is achieved using nudged elastic band simulations on a proxy MEAM potential for Mg designed to favor the pyramidal I over pyramidal II. The minimum energy transition path for pyramidal I double cross-slip is found to initiate with cross-slip onto a pyramidal II plane followed by cross-slip onto a pyramidal I plane parallel to the original pyramidal I plane. A previous mechanistic model for ductility is then extended to higher solute concentrations where pyramidal I is favorable. The model predicts an upper limit of solute concentrations beyond which ductility again becomes poor in Mg alloys. The model predictions are consistent with limited experiments of Mg-RE alloys at high concentrations and motivate further experimental studies in the high concentration regime. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

PERGAMON-ELSEVIER SCIENCE LTD2020

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